1. Field of the Invention
This invention is in the field of night vision goggles (NVG) or other optical devices which require the user to adjust an eyepiece in order to focus an image on the eye retina.
Night vision goggles are electro-optical instruments that have an adjustment for the focus of the intensified image at the viewing screen. The focus adjustment is made by adjusting the eyepiece lens which allows individuals with different eye focus characteristics to see the intensified image clearly, much like the adjustment for the eye found on microscopes and telescopes. For the purpose of an NVG, the eyepiece lens adjustment is required to provide the eye with a clear image of the intensified image on the viewing screen. The eyepiece focus in the NVG is distinct from the input focus of the optical system in that an image of the distant scene is on the photo cathode (FIG. 1).
It is known that NVG users have difficulty in correctly focusing the eyepiece lens. It is believed that the reasons that make it difficult to focus the adjustable eyepieces on most optical instruments are similar to the reasons for the difficulties associated with NVG applications.
For most optical instruments, inaccurate eyepiece lens focus, at worst, results in significant eye strain, headache, and diplopia; however, with the NVG, the eye strain, headache and related performance decrement are introduced into life and death situations. There is a consensus among Army, Air Force and Navy personnel that focusing of NVG devices produces a significant problem to which there is no consensus on solution.
2. The Prior Art
In the NVG prior art, the eyepiece lens is focused on the viewing screen. The user must adjust the focus for minimum smear or fuzziness, or for maximum contrast between different areas of the screen.
One solution to the focusing problems has been to completely eliminate the adjustable eye focus piece.
The use of small holes or slits for shielding the eye is known. British Patent 521,138 shows a device which has opaque eyepieces and small holes or slits in the centers for admission of a limited amount of light. In this device, the small hole or slit operates as a diaphragm, and not as a focusing aid.
The prior art also includes the work done by Christian Scheiner, reported in the year 1619. The Scheiner principle is illustrated in FIGS. 2-6. FIG. 2 shows a bundle of light 10 from an infinitely far point source that enters the lens 12 from the left and the lens focusing the bundle to the point 14 on the right. FIG. 3 repeats FIG. 2 with changes. Most of the lens has been made opaque, except for two small holes, i and ii. The opaque lens 12 blocks all light except that which passes through the holes. The holes are used to separate two pencils of light 16, 18 from the whole bundle 10. One light pencil passes through hole i and one passes through hole ii. These two pencils of light come together at the point of focus 14 of the lens, where the hole bundle would have been focused if the lens had been transparent.
FIGS. 4, 5 and 6 illustrate the effect the holes (i, ii) have when a focusing screen 20 is placed close to the lens (FIG. 4), at the focal plane (FIG. 5) or beyond the focal plane (FIG. 6).
FIG. 4 illustrates the situation when the screen is positioned at point A. The two bundles of light 16, 18 are isolated by a pair of holes i and ii which are projected onto the screen 20 at plane A. In this position, the screen has two separate images, the upper from hole i and lower from hole ii.
FIG. 5 illustrates the situation where the screen 20 is positioned at point B. The two bundles 16, 18 of light isolated by a pair of holes i and ii are projected onto the screen at plane B. If a screen is viewed, a single image is seen because of super imposed images from holes i and ii.
FIG. 6 illustrates a situation where the screen 20 is moved beyond the focal plane, 14 to a point C. This screen will be illuminated by two separate images, a lower image from hole i and an upper image from hole ii. This is the reverse of FIG. 4.